The invention relates to a method for the measurement of diffusivity or flow velocity. The invention also relates to a microsensor for measurement of diffusivity or flow velocity, which microsensor has a reservoir provided with a passage- or transport area and at least one transducer.
Several principles are known for the measurement of flow velocity in fluids. For example the induction principle (Faraday-principle), the skin friction/hot wire principle and the Laser-Doppler-principle. All these principles make use of measuring devices with considerable dimensions. Consequently, the medium where the measurement takes place have to fulfill specific requirements such as sufficient flow and volume.
The U.S. Pat. No. 5,339,694 describes a sensor for measurement of flow in a matrix, more precisely measuring migration of groundwater. The sensor consists of a circular pipe having a liquid permeable diaphragm. Outside the cylinder are transducers for measurement of conductivity. The cylinder contains a saline solution with a conductivity that differs from that of groundwater. When the sensor is placed in a water saturated stratum, salt ions from the cylinder will diffuse through the permeable diaphragm into the stratum with groundwater. A change in the conductivity in an area outside the cylinder is registered by the sensors outside the cylinder, whereby the flow velocity can be determined.
This sensor makes use of a suitable principle for the determination of small flow velocities through bigger volumes of a matrix. However, due to the dimensions of the sensor, it is not possible to use it in smaller volumes such as layer transitions in fluids or in vessels in vegetable or animal tissues, in filters or biofilm.